Method and Device for Determining the Motor Constant of an Electric Motor

ABSTRACT

The invention concerns a method and a device for determining the motor moment constant k M  of an electric motor by measuring motor parameters on the running motor. For reduction of the previously considerable measuring effort it is proposed that firstly the generator voltage U EMK  produced by the motor is measured, and in that the motor moment constant k M  is calculated by division of the generator voltage U EMK  and the speed of rotation f Mot  of the motor, taking into consideration at least one further constant. The method and the device are suitable for DC motors and for 3-phase synchronous motors.

The invention relates to a method and a device for the determination of the motor constant of an electric motor by measuring motor parameters on the running motor.

To date in electronic motor controls there are used for the motor moment regulation and/or motor moment restriction motor moment constants k_(M). With the aid of this relationship:

M _(Mot) =k _(M) ·I _(Mot)  (1)

the produced inner motor moment (without friction losses, therefore inner moment) can be determined on the basis of the measured or regulated motor current I_(Mot).

As a rule, thereby the motor moment constant is determined by measurement techniques over a statistically sufficient number of motors on a motor test bed and then stored as constant in the motor control.

For the determination of the motor moment constant there is employed as a rule a torque measurement device and a current measurement device and the motor moment constant is then calculated via the formula

$\begin{matrix} {k_{M} = \frac{M_{Mot}}{I_{Mot}}} & (2) \end{matrix}$

It is here clear that a great measuring outlay is necessary for the determination of the motor moment constants and only (depending on application) a mean, maximum or minimum value of the motor moment constant can be employed for the motor control, insofar as no calibration is carried out for each motor.

The invention is based on the object of reducing the measuring effort for the determination of the motor moment constant. Furthermore the possibility is to be opened up of making a self-calibrating motor control possible.

The invention is achieved by means of the method indicated in claim 1 and the device indicated in claim 6.

A concrete realization of the method in accordance with the invention can consist, in accordance with claim 2, in that the generator voltage U_(EMK) produced by the motor is measured, and in that the motor moment constant k_(M) is calculated according to the following formulae.

For a DC motor

$\begin{matrix} {k_{M} = \frac{U_{EMK}}{2{\pi \cdot f_{Mot}}}} & (3) \end{matrix}$

and for a 3-phase synchronous motor

$\begin{matrix} {k_{M} = {\frac{U_{EMK}}{2{\pi \cdot f_{Mot}}} \cdot \sqrt{3}}} & (4) \end{matrix}$

The above formulae arise as follows:

There applies:

P _(Mech) =M·ψ  (5)

Where P_(Mech) is the mechanical motor power, M the motor moment and ω the angular frequency.

There applies further for an ideal loss-free DC motor

P _(el) =U _(EMK) ·I _(Mol)  (6)

and for an ideal loss-free 3-phase synchronous motor

P ^(el) =U _(EMK, phph) ·I _(Mot)·√{square root over (3)}  (7)

Where P_(el) is the electrical motor power, U_(EMK) the generator voltage produced by the motor and I_(Mot) the generator current produced by the motor.

There further applies for both motor types

P_(Mech)=P_(el)  (8)

Equating formulae (5) and (6) one then obtains for the DC motor

U _(EMK) ·I _(Mot) =M·2π·f _(Mot)  (9)

And equating formulae (5) and (7) for the 3-phase synchronous motor

U _(EMK, phph) ·I _(Mot, phph) ·√{square root over (3)} =M·2π·f _(Mot)  (10)

in which f_(Mot) is the speed of rotation of the motor which also is described as revolution frequency or rotational frequency. The speed of rotation is a physical parameter with the dimension 1/time. As a rule, it is indicated for motors as revolutions/minute.

From formula (9) there is provided for the DC motor:

$\begin{matrix} {M = {\frac{U_{EMK}}{2{\pi \cdot f_{Mot}}} \cdot I_{Mot}}} & (11) \end{matrix}$

From formula (10) there is provided for the 3-phase synchronous motor

$\begin{matrix} {M = {\frac{U_{{EMK},{phph}}}{2{\pi \cdot f_{Mot}}} \cdot I_{{Mot},{phph}} \cdot \sqrt{3}}} & (12) \end{matrix}$

If one combines formulae (2) and (11), formula (3) thus arises.

If one combines formulae (2) and (12) and sets cos phi=1, formula (4) arises. Both could be proven.

Configurations of the method in accordance with the invention directed specifically to DC motors and 3-phase synchronous motors are subject of claims 2-5.

A first configuration of the solution in accordance with the invention for a DC motor can consist in that the motor is taken to a predetermined speed of rotation, externally driven, before the measuring of the generator voltage, which is then used for the calculation of the motor moment constant. With this variant the speed of rotation is fixed and need not be measured first; only the generator voltage produced by the motor must still be measured.

An alternative configuration of the invention, also for a DC motor, can consist in that by applying an operating voltage the motor is started before the measuring, that the operating voltage is then switched off, and that the measuring of the generator voltage is effected after fading away of the inductive operating current and at the same time the speed of rotation is measured via an external speed of rotation measuring device. With this variant an external speed of rotation measuring device is also required besides the measuring device for the generator voltage; however, the external drive is not needed.

A further configuration of the invention for a 3-phase synchronous motor can consist in that initially the motor is started without load by applying an operating voltage, in that then the operating voltage is switched off and after fading away of the inductive operating current the generator voltage is measured, wherein the speed of rotation is provided from the cycle duration of the generator voltage produced. With this variant there is likewise needed only a measurement device for the generator voltage since the rotational frequency, as mentioned, is provided from the cycle duration of the produced generator voltage.

With none of the above described variants is a torque measurement device still required, which previously was necessary in every case and in equipment technology terms is rather complex.

To be able to realize highly exact moment control, the open-circuit current must be determined. This open-circuit current contains motor internal losses (e.g. relating to magnetization and friction losses) which then take part as offset in the calculation of the motor moment.

By means of the method an intelligent control can be realized with which the connected motor can be measured so that it is then able to deliver very exactly the desired moment. Through this a calibration with an external moment test device can be forgone in most cases.

Exemplary embodiments of the invention will be described below with reference to the drawings. There is shown:

FIG. 1 a circuit for the determination of the motor moment constant of a DC motor;

FIG. 2 an alternative circuit to FIG. 1 for the determination of the motor moment constant;

FIG. 3 a circuit for the determination of the motor moment constant of a 3-phase synchronous motor.

In FIG. 1 a DC motor 3 is provided with operating voltage from a DC current supply system 1 via a double-throw switch 2. The DC motor 3 is started in the illustrated switching position of the double-throw switch 2. A speed of rotation measuring device 4 sits on the motor shaft of the DC motor 3, with which the speed of rotation f_(Mot) of the DC motor 3 can be determined.

After the DC motor 3 has started, at first it is separated from the DC current supply system 1 with the double-throw switch 2 until the inductive operating current has faded away. The double-throw switch 2 is then switched over into the switching position indicated by broken lines. In this switching position a voltage measurement device 5 is connected with the terminals of the DC motor 3 which measures the generator voltage U_(EMK) of the motor 3. The measurement result is delivered to a computer 6. The measurement result of speed of rotation measuring device 4 is further delivered to the computer 6 via the double-throw switch 2. The computer 6 determines the motor moment constant k_(M) from the generator voltage U_(EMK) and from the speed of rotation f_(Mot) by division according to the formula (3) given in the introduction.

With the variant shown in FIG. 2 the DC motor 13 is started not through its own force but by means of a mechanical coupling with an auxiliary motor 17 which in the present case is a 3-phase AC motor which is fed by a 3-phase AC network 11. Since the 3-phase AC network has a fixed known frequency, this can be delivered to a computer 16. In addition, there is supplied to the computer 16 a value of the generator voltage which is determined by a voltage measuring device 15, wherein the voltage measuring device 15 is connected to the electrical terminals of the DC motor 13. In turn the computer 16 calculates, through division of the generator voltage U_(EMK) and the speed of rotation f_(Mot) of the motor according to the formula (3) given in the introduction, the motor moment constant k_(M).

The circuit shown in FIG. 3 for the determination of the moment constant k_(M) is designed for a 3-phase synchronous motor 23. This is firstly connected via a double-throw switch 22 to a 3-phase AC network 21. After the motor 23 has started and has reached its synchronous speed of rotation, the double-throw switch 22 is firstly switched over into the middle position in which the motor 23 is separated from the network 21. The double-throw switch 22 remains in this position until the inductive operating current in the motor 23 has faded away. The double-throw switch 22 is then switched over into the lower switching position indicated by broken lines. In this switching position two of the three terminals of the motor 23 are connected with a voltage measurement device 25 which measures the generator voltage 25 produced by the now idling motor 23 and delivers the result to a computer 26. In addition, there is delivered to the computer 26 as further value the speed of rotation of the motor which is known from the construction of the motor 23 and the frequency of the 3-phase AC network 21. The computer then calculates from the values delivered thereto the motor moment constant k_(M) by division according to the formula (4) given in the introduction.

With the measuring means for the generator voltage of the motor and its speed of rotation, and the device containing the computer for the determination of the motor moment constants k_(M), motor control electronics can be realized which can automatically measure the motors in accordance with the described method. In this way production variations of the motor can compensated and a very exact moment control be realized without a calibration being required. However, the device finds application not only in production but also in the laboratory in order to measure the motors without a complex test bed being necessary.

Furthermore there is also the possibility of integrating the device in accordance with the invention into a motor control which measures the connected motor, as described above, and uses the measurement results—that is, the motor moment constant here obtained—for the more exact control of the motor, in particular for a control/regulation of the torque. 

1. Method for determining the motor moment constant k_(M) of an electric motor by measuring motor parameters on the running motor comprising: producing a generator voltage U_(EMK) by a running electric motor and a speed of rotation f_(Mot) of the running electric motor; and calculating a motor moment constant k_(M) for one or more of a DC motor and a 3-phase synchronous motor using the produced generator voltage U_(EMK) and the speed of rotation f_(Mot).
 2. Method according to claim 1, comprising measuring the generator voltage U_(EMK) produced by the running electric motor and calculating the motor moment constant is k_(M) according to the following formulae for the DC motor $k_{M} = \frac{U_{EMK}}{2{\pi \cdot f_{Mot}}}$ and for the 3-phase synchronous motor $k_{M} = {\frac{U_{EMK}}{2{\pi \cdot f_{Mot}}} \cdot \sqrt{3}}$
 3. Method according to claim 2 wherein the motor is an externally driven DC motor, the method comprising before measuring the generator voltage U_(EMK), bringing the motor to a predetermined speed of rotation f_(Mot) that is used for the calculation.
 4. Method according to claim 2 wherein the motor is a DC motor, comprising, before measuring, starting the motor by applying an operating voltage to produce an inductive operating current, then switching off the operating voltage, and then measuring of the generator voltage U_(EMK) after the inductive operating current fades away and at the same time measuring the speed of rotation f_(Mot) via an external speed of rotation measuring device.
 5. Method according to claim 2 wherein the motor is a 3-phase synchronous motor, comprising starting the motor by applying an operating voltage to produce an inductive operating current, then switching off the operating voltage and, after the inductive operating current fades away, measuring the generator voltage U_(EMK), wherein the speed of rotation f_(Mot) is provided by the cycle duration of the generator voltage U_(EMK) produced.
 6. Device for determining the motor moment constant k_(M) of an electric motor, characterized by comprising: (a) measurement means for the generator voltage U_(EMK) produced by the motor, (b) measurement means for the speed of rotation f_(Mot) of the motor, and (c) a computer to which the measurement results of the measurement means are delivered and which calculates the motor moment constant k_(M) therefrom.
 7. (canceled)
 8. Method for using a motor moment constant k_(M) for controlling a running electric motor comprising: producing a generator voltage U_(EMK) by a running electric motor and a speed of rotation f_(Mot) of the running electric motor; calculating a motor moment constant k_(M) for one or more of a DC motor and a 3-phase synchronous motor using the produced generator voltage U_(EMK) and the speed of rotation f_(Mot); and controlling the running electric motor by using the calculated motor moment constant.
 9. Method according to claim 8, comprising measuring the generator voltage U_(EMK) produced by the running motor and calculating the motor moment constant is k_(M) according to the following formulae for the DC motor: $k_{M} = \frac{U_{EMK}}{2{\pi \cdot f_{Mot}}}$ and for the 3-phase synchronous motor $k_{M} = {\frac{U_{EMK}}{2{\pi \cdot f_{Mot}}} \cdot \sqrt{3}}$
 10. Method according to claim 9, wherein the motor is an externally driven DC motor and the method comprises, before measuring the generator voltage U_(EMK), bringing the motor to a predetermined speed of rotation f_(Mot) that is used for the calculation.
 11. Method according to claim 9 for the DC, wherein the motor is a DC motor and the method comprises before measuring, starting the motor by applying an operating voltage and an inductive operating current, then switching off the operating voltage, and measuring the generator voltage U_(EMK) after the inductive operating current fades away and at the same time measuring the speed of rotation f_(Mot) via an external speed of rotation measuring device.
 12. Method according to claim 9 wherein the motor is a 3-phase synchronous motor, comprising starting the motor by applying an operating voltage and an inductive operating current, then switching off the operating voltage and after the inductive operating current fades away measuring the generator voltage U_(EMK), wherein the speed of rotation f_(Mot) is provided by the cycle duration of the generator voltage U_(EMK) produced. 